Andrea Fratalocchi

Publications (32)58.73 Total impact

• Article: Heterogeneous shear elasticity of glasses: the origin of the boson peak.

  Alessia Marruzzo, Walter Schirmacher, Andrea Fratalocchi, Giancarlo Ruocco
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  ABSTRACT: The local elasticity of glasses is known to be inhomogeneous on a microscopic scale compared to that of crystalline materials. Their vibrational spectrum strongly deviates from that expected from Debye's elasticity theory: The density of states deviates from Debye's law, the sound velocity shows a negative dispersion in the boson-peak frequency regime and there is a strong increase of the sound attenuation near the boson-peak frequency. By comparing a mean-field theory of shear-elastic heterogeneity with a large-scale simulation of a soft-sphere glass we demonstrate that the observed anomalies in glasses are caused by elastic heterogeneity. By observing that the macroscopic bulk modulus is frequency independent we show that the boson-peak-related vibrational anomalies are predominantly due to the spatially fluctuating microscopic shear stresses. It is demonstrated that the boson-peak arises from the steep increase of the sound attenuation at a frequency which marks the transition from wave-like excitations to disorder-dominated ones.
  Scientific Reports 03/2013; 3:1407.
• Article: Double shock dynamics induced by the saturation of defocusing nonlinearities.

  Matteo Crosta, Andrea Fratalocchi, Stefano Trillo
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  ABSTRACT: We show that the saturation of defocusing nonlinearities leads to qualitative changes in the onset of wave breaking, determining double shock formation whose regularization occurs in terms of antidark solitons. In a given material, the crossover between different regimes can be controlled by changing the input intensity.
  Optics Letters 05/2012; 37(10):1634-6. · 3.40 Impact Factor
• Source

  Available from: Andrea Armaroli

  Article: Suppression of transverse instabilities of dark solitons and their dispersive shock waves

  Andrea Armaroli, Andrea Fratalocchi, Stefano Trillo
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  ABSTRACT: We investigate the impact of nonlocality, owing to diffusive behavior, on transverse instabilities of a dark stripe propagating in a defocusing cubic medium. The nonlocal response turns out to have a strongly stabilizing effect both in the case of a single soliton input and in the regime where dispersive shock waves develop "multisoliton regime". Such conclusions are supported by the linear stability analysis and numerical simulation of the propagation.
  01/2012;
• Source

  Available from: ArXiv

  Article: Molecular imaging with X-ray free electron lasers: dream or reality?

  Andrea Fratalocchi, Giancarlo Ruocco
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  ABSTRACT: X-ray Free Electron Lasers (XFEL) are revolutionary photons sources, whose ultrashort, brilliant pulses are expected to allow single molecule diffraction experiments providing structural information on the atomic length scale. This ultimate goal, however, is currently hampered by several challenging questions basically concerning sample damage, Coulomb explosion and the role of nonlinearity. By employing an original \emph{ab-initio} approach, as well as exceptional resources of parallel computing, we address these issues showing that accurate XFEL-based single molecule imaging will be only possible with ultrashort pulses of half of femtosecond, due to significant radiation damage and the formation of preferred multi-soliton clusters which reshape the overall electronic density of the molecular system at the femtosecond scale. Comment: 22 pages, 5 Figures
  09/2010;
• Conference Proceeding: Control of Dispersive Shock Dynamics Developing from Dark Waveforms

  Stefano Trillo, Andrea Armaroli, Stefania Malaguti, Andrea Fratalocchi
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  ABSTRACT: We investigate the dynamics of 1-D dispersive shock waves generated from smooth dark waveforms in the weakly-dispersive limit. Different forms of control of their dynamics as well as their stabilization against transverse instabilities are discussed.
  Nonlinear Photonics (NP), Karlsruhe, Germany; 06/2010
• Chapter: Nonlinear Optical Waves in Liquid Crystalline Lattices

  Gaetano Assanto, Andrea Fratalocchi
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  ABSTRACT: Liquid crystals (LC) are molecular dielectrics encompassing several properties of both liquids and solids; in particular, they are often characterized by an order parameter which can be employed to distinguish among possible LC phases. In the nematic phase, liquid crystals show a significant degree of orientational order, their elongated organic molecules being aligned in a mean direction in space, as described by a vectorial field n called director. Sincemost nematics are derivative of benzene, they feature “cigar-like” molecules; hence, the macroscopic system can be regarded as an optically uniaxial crystalline fluid. The dielectric tensor \overleftrightarrowe( r ) \overleftrightarrow{\varepsilon \left( r \right)} , describing the optical polarization of the medium, can be expressed as \overleftrightarrowe = \overleftrightarrowRf \overleftrightarrowe NLC \overleftrightarrowR \overleftrightarrow{\varepsilon} = \overleftrightarrow{R}^\dag \cdot \overleftrightarrow{\varepsilon} _{{\rm NLC}} \cdot \overleftrightarrow{R} , with \overleftrightarroweNLC = [ e^ , e^ , e|| ] I, Iij = dij \overleftrightarrow{\varepsilon}_{{\rm NLC}} = \left[ \varepsilon_\bot , \varepsilon_\bot , \varepsilon_\| \right] I, I_{ij} = \delta_{ij } ( dij \delta_{ij } is the Kronecker delta) and \overleftrightarrowR(n) \overleftrightarrow{R}(n) a rotation tensor. The steady-state director configuration is obtained as an extremal point of the action integral I = \smallint L dxdydz \mathcal{I} = \smallint \mathcal{L} d\rm{x}d\rm{y}d\rm{z} , whose density L \mathcal{L} defines the energy spent by the molecular system to hold a specific director configuration (Frank freeenergy formulation) [1]. The energy density L \mathcal{L} can be further expanded into elastic Lel \mathcal{L}_{\rm{el}} and electromagnetic Lem \mathcal{L}_{\rm{em}} terms: L = Lel + Lem \mathcal{L} = \mathcal{L}_{\rm{el}} + \mathcal{L}_{\rm{em}} . The contribution Lel \mathcal{L}_{\rm{el}} can be evaluated in the framework of the elastic continuum theory and, in the single constant approximation [2], reads: Lrmel = \frac12K[ ( Ñn )2 + ( n Ñn )2 ],\mathcal{L}_{rm{el}} = \frac{1}{2}K\left[ {\left( {\nabla \cdot n} \right)^2 + \left( {n \cdot \nabla \times n} \right)^2 } \right], with K accounting for elastic deformations ([K] = N). The electromagnetic contribution can be calculated by considering that the electric field induces dipoles on the nematic liquid crystal (NLC) molecules; the latter are then subjected to a torque and change their angular orientation towards a minimum energy configuration (e.g., parallel to the applied field). The contribution describing such reorientation process is [2]: Lrmem = -\fracDe2 n E ,\mathcal{L}_{rm{em}} = -\frac{{\Delta \varepsilon }}{2}\left\langle {n \cdot E} \right\rangle , being De = e|| - e^ \Delta \varepsilon = \varepsilon_{\|} - \varepsilon_{\bot} the NLC birefringence and ¼ \left\langle \ldots \right\rangle denoting a square time average. The balance between field-induced reorientation and elastic interactions gives rise to the steady state distribution n, found as an extremal of the action integral dL = 0 \delta \mathcal{L} = 0 .
  03/2010: pages 21-35;
• Source

  Available from: Stefano Trillo

  Article: Observation of a gradient catastrophe generating solitons.

  Claudio Conti, Andrea Fratalocchi, Marco Peccianti, Giancarlo Ruocco, Stefano Trillo
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  ABSTRACT: We investigate the propagation of a dark beam in a defocusing medium in the strong nonlinear regime. We observe for the first time a shock fan filled with noninteracting one-dimensional gray solitons that emanates from a gradient catastrophe developing around a null of the optical intensity. This scenario turns out to be very robust, persisting also when the material nonlocal response averages the nonlinearity over dimensions much larger than the emerging soliton filaments.
  Physical Review Letters 03/2009; 102(8):083902. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Ultrashort pulse propagation and the Anderson localization

  Silvia Gentilini, Andrea Fratalocchi, Luca Angelani, Giancarlo Ruocco, Claudio Conti
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  ABSTRACT: We investigate the dynamics of a 10 fs light pulse propagating in a random medium by the direct solution of the 3D Maxwell equations. Our approach employs molecular dynamics to generate a distribution of spherical scatterers and a parallel finite-difference time-domain code for the vectorial wave propagation. We calculate the disorder-averaged energy velocity and the decay time of the transmitted pulse Versus the localization length for an increasing refractive index. Comment: 3 pages, 5 figures
  10/2008;
• Source

  Available from: Stefano Trillo

  Article: Free-energy transition in a gas of non-interacting nonlinear wave-particles

  Andrea Fratalocchi, Claudio Conti, Giancarlo Ruocco, Stefano Trillo
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  ABSTRACT: We investigate the dynamics of a gas of non-interacting particle-like soliton waves, demonstrating that phase transitions originate from their collective behavior. This is predicted by solving exactly the nonlinear equations and by employing methods of the statistical mechanics of chaos. In particular, we show that a suitable free energy undergoes a metamorphosis as the input excitation is increased, thereby developing a first order phase transition whose measurable manifestation is the formation of shock waves. This demonstrates that even the simplest phase-space dynamics, involving independent (uncoupled) degrees of freedom, can sustain critical phenomena. Comment: 4 pages, 3 figures
  08/2008;
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  Available from: ArXiv

  Article: Dynamic light diffusion, Anderson localization and lasing in disordered inverted opals: 3D ab-initio Maxwell-Bloch computation

  Claudio Conti, Andrea Fratalocchi
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  ABSTRACT: We report on 3D time-domain parallel simulations of Anderson localization of light in inverted disordered opals displaying a complete photonic band-gap. We investigate dynamic diffusion processes induced by femtosecond laser excitations, calculate the diffusion constant and the decay-time distribution versus the strength of the disorder. We report evidence of the transition from delocalized Bloch oscillations to strongly localized resonances in self-starting laser processes.
  03/2008;
• Article: Nonlinearly controlled angular momentum of soliton clusters.

  Andrea Fratalocchi, Armando Piccardi, Marco Peccianti, Gaetano Assanto
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  ABSTRACT: We demonstrate an original approach, to the best of our knowledge, to acquire nonlinear control over the angular momentum of a cluster of solitary waves. We show that the angular momentum can be adjusted by acting on the global excitation of the system. The effect is verified in liquid crystals by observing power-dependent rotation of a two-soliton cluster.
  Optics Letters 07/2007; 32(11):1447-9. · 3.40 Impact Factor
• Article: Symmetry-breaking instabilities in perturbed optical lattices: Nonlinear nonreciprocity and macroscopic self-trapping

  Andrea Fratalocchi, Gaetano Assanto
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  ABSTRACT: We develop an asymptotic analysis of nonlinear energy propagation in lattices subject to slowly varying perturbations, investigating symmetry breaking and its effects. We derive a general set of evolution equations and study them by using catastrophe theory, revealing a wealth of system dynamics. Below a power threshold, symmetry breaking drives nonreciprocal oscillations; beyond that, symmetry breaking yields an effect of “macroscopic” self-trapping, which supports a self-maintained energy imbalance between Bloch bands. We numerically verify the theoretical results and discuss their possible implementation in waveguide arrays.
  Phys. Rev. A. 06/2007; 75(6).
• Article: Nonlinear management of the angular momentum of soliton clusters: Theory and experiment

  Andrea Fratalocchi, Armando Piccardi, Marco Peccianti, Gaetano Assanto
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  ABSTRACT: We demonstrate, both theoretically and experimentally, how to acquire nonlinear control over the angular momentum of a cluster of solitary waves. Our results, stemming from a universal theoretical model, show that the angular momentum can be adjusted by acting on the global energy input in the system. The phenomenon is experimentally ascertained in nematic liquid crystals by observing a power-dependent rotation of a two-soliton ensemble.
  Physical Review A 06/2007; 75(6). · 2.88 Impact Factor
• Article: Nonlinear adiabatic evolution and emission of coherent Bloch waves in optical lattices

  Andrea Fratalocchi, Gaetano Assanto
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  ABSTRACT: We investigate energy propagation in a perturbed optical lattice, studying the nonlinear adiabatic evolution of a light beam. Through an asymptotic model, we demonstrate that: (i) energy propagating in the adiabatic regime visualizes the dispersion relation of the unperturbed lattice; (ii) the adiabatic evolution can be broken by nonlinearity, giving rise to tunneling between Bloch bands; (iii) the tunneling is accompanied by nonlinear emission of one—or more—coherent Bloch waves, the properties of which can be completely controlled by linear and/or nonlinear parameters. We verify the analytical results against numerical simulations and indicate possible experimental implementations.
  Phys. Rev. A. 01/2007; 75(1).
• Source

  Available from: Marco Peccianti

  Article: Nonlinear management of the angular momentum of soliton clusters

  Andrea Fratalocchi, Armando Piccardi, Marco Peccianti, Gaetano Assanto
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  ABSTRACT: We demonstrate an original approach to acquire nonlinear control over the angular momentum of a cluster of solitary waves. Our model, derived from a general description of nonlinear energy propagation in dispersive media, shows that the cluster angular momentum can be adjusted by acting on the global energy input into the system. The phenomenon is experimentally verified in liquid crystals by observing power-dependent rotation of a two-soliton cluster. Comment: 4 pages, 3 figures
  12/2006;
• Source

  Available from: uniroma3.it

  Article: Dispersion spectroscopy of photonic lattices.

  Andrea Fratalocchi, Gaetano Assanto
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  ABSTRACT: We theoretically demonstrate how to acquire the full bandgap spectrum of a photonic lattice of arbitrary profile. Focusing on the 1D case in the presence of a linear refractive index acceleration and employing a multiscale analysis, we show that each photonic band can be directly mapped by the light evolution in the lattice, whereas the size of each gap corresponds to the tunneling rate. We verify the analytical results with numerical simulations and discuss experimental realizations of this technique for dispersive spectroscopy.
  Optics Letters 12/2006; 31(22):3351-3. · 3.40 Impact Factor
• Article: Discrete Light Propagation and Self-Localization in Voltage-Controlled Arrays of Channel Waveguides in Undoped Nematic Liquid Crystals

  Andrea Fratalocchi, Gaetano Assanto, Katarzyna A. Brzdąkiewicz, Mirosław A. Karpierz
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  ABSTRACT: We review linear and nonlinear light propagation in a voltage-tunable liquid crystalline waveguide array. We report on discrete diffraction, discrete soliton generation, all-optical beam steering, multiband vector breathers and Bloch oscillations. The predictions, based on full numerical BPM simulations, are in excellent agreement with the experimental results.
  Molecular Crystals and Liquid Crystals 09/2006; 453(2006: pp. 191–202):191-202. · 0.58 Impact Factor
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  Available from: uniroma3.it

  Article: Light propagation through a nonlinear defect: symmetry breaking and controlled soliton emission.

  Andrea Fratalocchi, Gaetano Assanto
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  ABSTRACT: We investigate the emission of solitons at a nonlinear longitudinal defect. We discuss the basic physics and introduce a novel approach to achieve complete nonlinear control of the process. Theoretical results are confirmed by numerical simulations.
  Optics Letters 06/2006; 31(10):1489-91. · 3.40 Impact Factor
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  Available from: uniroma3.it

  Article: Governing soliton splitting in one-dimensional lattices.

  Andrea Fratalocchi, Gaetano Assanto
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  ABSTRACT: We investigate discrete light dynamics in the presence of a longitudinal defect of arbitrary extension, amplitude and position in a nonlinear waveguide array. We model and discuss the physics of the soliton-defect interaction, showing how to gain complete control over the system outcome for soliton-based data processing. We propose all-optical management in dye-doped liquid crystals.
  Physical Review E 05/2006; 73(4 Pt 2):046603. · 2.26 Impact Factor
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  Available from: uniroma3.it

  Article: Optically induced zener tunneling in one-dimensional lattices.

  Andrea Fratalocchi, Gaetano Assanto, Kasia A Brzdakiewicz, Mirek A Karpierz
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  ABSTRACT: We investigate Landau-Zener tunneling in one-dimensional liquid crystalline waveguide arrays by all-optical impression of acceleration with an additional beam. We derive the Zener model from the governing equations and demonstrate a novel approach to Floquet-Bloch band tunneling.
  Optics Letters 04/2006; 31(6):790-2. · 3.40 Impact Factor